1. Field of the Invention
The present invention relates to a method of forming a transparent conductive layer, which is uniform in film quality and thickness, by a sputtering process, a photoelectric conversion device using the transparent conductive layer, which has high reliability and can be easily mass-produced, and a manufacturing method for the photoelectric conversion device.
2. Description of the Related Art
Energy consumed by human beings at the present time greatly depends on thermal-power generation using fossil fuels such as petroleum products and coal, and nuclear power generation. But there are many problems in continuing to rely in the future on a large scale upon fossil fuels which generate carbon dioxide when burnt and which are responsible for warming of the atmosphere around the earth. Nuclear power is not totally free from a risk of exposure to radiation even during normal operation, as well as the possibility of accidental exposure. Therefore, solar power generation using photoelectric conversion devices, that barely affect the earth's environment, have received attention and further extensive use thereof is expected.
The current state in the field of solar power generation, however, faces several problems that hinder its widespread practical use.
Heretofore, solar cells for use in solar power generation have been formed of single-crystal or polycrystalline silicon. But these solar cells require not only a lot of energy and time for growth of crystals, but also complicated steps after the growth of crystals, and hence it is difficult to mass-produce the solar cells efficiently at a low cost. Meanwhile, the so-called thin film semiconductor solar cells using amorphous silicon (hereinafter referred to as a-Si) or compound semiconductors such as CdS and CuInSe.sub.2 have been intensively researched and developed. These thin film semiconductor solar cells can be manufactured by forming the necessary semiconductor layers on an inexpensive base plate made of glass, stainless steel, etc., with relatively simple steps, and hence have a good possibility that their production cost can be reduced. However, because of lower conversion efficiency than crystalline silicon solar cells and uncertain reliability for long-term use, the thin film semiconductor solar cells have not yet been used on a large scale. To solve such problems and improve performance of the thin film semiconductor solar cells, various measures have been proposed as follows.
One proposal is to provide a rear reflecting layer to increase reflectivity of light-at the front surface of the base plate for returning part of the incident light (such as sunlight), which has not been absorbed by the thin film semiconductor layers, to the thin film semiconductor layers, i.e., to more effectively utilize the incident light. To this end, in the case of using a base plate transparent to light that is absorbed by the semiconductor layers and introducing the light from the base plate side, an electrode is formed of a metal having high reflectivity, such as silver (Ag), aluminum (Al) and copper (Cu), on a surface of the thin film semiconductor layers after forming them. On the other hand, in the case of introducing the light from the surface of the thin film semiconductor layers, the semiconductor layers are formed after forming a similar metal layer on the base plate.
Further, by interposing a transparent layer having suitable optical properties between the metal layer and the semiconductor layers, reflectivity can be increased based on the multiple interference effect as shown in FIG. 2. FIGS. 2A and 2B show, by way of example, results of simulation tests and prove that reflectivity is increased by interposing zinc oxide (ZnO) as a transparent conductive layer between silicon and each of various metals.
The use of the transparent conductive layer is also effective in increasing reliability of the thin film solar cells. For example, Japanese Patent Publication No. 60-41878 discloses that the semiconductor and the metal are prevented from intermixing to form an alloy by the presence of the transparent layer. U.S. Pat. No. 4,532,372 and U.S. Pat. No. 4,598,306 disclose that, by using a transparent conductive layer which has a suitable resistance value, an excessive current is prevented from flowing between the electrodes even if the semiconductor layers are short-circuited somewhere.
Using only the above-described measures, however, desired requirements of a photoelectric conversion device cannot be obtained. Thus, solar cells using thin film semiconductors as a photoelectric conversion device are now in a state such that there is a good possibility for producing such solar cells at a low cost, but they have not yet been used on a large scale for solar power generation.